The present invention relates to a rotation sensor that outputs a three-phase signals corresponding to the rotational angle of a rotor and to a rotational angle detection apparatus employing the rotation sensor.
Conventionally, a power steering apparatus for a vehicle has rotational angle detection apparatuses mounted in a steering shaft and an electric motor. FIG. 12 shows an example of a rotational angle detection apparatus that detects the rotational angle of a rotary shaft such as a steering shaft.
As illustrated in FIG. 12, the rotational angle detection apparatus includes a resolver 2 serving as a rotation sensor that outputs three-phase voltage signals Va, Vb, and Vc corresponding to the rotational angle of a rotary shaft 1 and a controller 3 for detecting the rotational angle of the rotary shaft 1 based on the signals Va to Vc output from the resolver 2. In the case described herein, the multiplication factor for the angle of the resolver 2 is set to 1.
The resolver 2 is configured by a rotor 20, which is formed by a magnetic body that rotates integrally with the rotary shaft 1, and a stator 21, which is spaced from the rotor 20 at a predetermined interval. The stator 21 has an excitation coil 22, a first resolver coil 23, a second resolver coil 24, and a third resolver coil 25. The first to third resolver coils 23 to 25 are spaced apart each at an interval of 120° in an electric angle phase about the rotational center ◯ of the rotor 20 in a circumferential direction (the direction indicated by arrow R1 in FIG. 12). Corresponding ends of the resolver coils 23 to 25 are electrically connected together. The other ends of the resolver coils 23 to 25 are electrically connected to corresponding output terminals Ta, Tb, Tc through associated signal lines 23a, 24a, 25a. The excitation coil 22 receives AC voltage from an oscillating circuit 4, which is mounted in the rotational angle detection apparatus, to generate an alternating magnetic field. In the resolver 2, an alternating magnetic field generated by the excitation coil 22 is provided to the first to third resolver coils 23 to 25 through the rotor 20. This induces the voltages (a1), (a2), and (a3) described below each having amplitude varied in a sinusoidal manner with respect to the rotational angle (the electric angle) θe of the rotor 20 in the first to third resolvers 23, 24, and 25, respectively, through electromagnetic induction. In this case, the oscillating circuit 4 supplies AC voltage Vr (Vr=E×sin(ωt)) to the excitation coil 22 (E represents the amplitude, ω represents the angular frequency, and t represents the time). Further, K represents the transformation ratio.
(a1) Voltage Va (Va=K×E×sin(θe)×sin(ωt)) is induced in the first resolver coil 23.
(a2) Voltage Vb (Vb=K×E×sin(θe+120°)×sin(ωt)) is induced in the second resolver coil 24.
(a3) Voltage Vc (Vc=K×E×sin(θe+240°)×sin(ωt)) is induced in the third resolver coil 25.
The voltages Va to Vc, which are induced in the first to third resolver coils 23 to 25, are output from the corresponding output terminals Ta to Tc of the resolver 2 and received by the controller 3.
The controller 3 performs signal processing to extract amplitude components from the signals Va to Vc output from the resolver 2. The controller 3 thus obtains an amplitude value Sa (Sa=K×E×sin(θe)) from the output signal Va, an amplitude value Sb (Sb=K×E×sin(θe+120°)) from the output signal Vb, and an amplitude value Sc (Sc=K×E×sin(θe+240°)) from the output signal Vc. FIG. 13 is a graph representing the relationship between the output signal amplitude values Sa to Sc and the mechanical angle θ of the rotor 20 in which the output signal amplitude values Sa to Sc are plotted along the axis of the ordinate and the mechanical angle θ is plotted along the axis of the abscissas. FIG. 13 illustrates a case in which the multiplication factor of angle of the resolver 2 is set to four times (4×), the electric potential difference Vpp (Vpp=2×E) between the peaks of the AC voltage Vr provided to the excitation coil 22 is set to 4[V], and the transformation ratio K is set to 0.2.
The controller 3 calculates the electric angle θe of the rotor 20 in three different manners using the expressions (1) to (3) described below, from the output signal amplitude values Se to Sc varied in the manners illustrated in FIG. 13. The electric angles calculated using the expressions (1), (2), and (3) are represented as the electric angles θe1, θe2, and θe3, respectively. Each of the expressions (1) to (3) transforms the corresponding one of the output signal amplitude values Sa to Sc to the relationship between the sine value and the cosine value and obtains the electric angle of the rotor 20 from the arc sine value of the output signal amplitude value Se to Sc.θe1=tan−1((√3×Sa)/−2×Sb−Sa))  (1)θe2=tan−1((√3×Sb)/(−2×Sc−Sb))−120°  (2)θe3=tan−1((√3×Sc)/(−2×Sa−Sc))−240°  (3)
FIG. 14 is a graph representing the relationship between the electric angles θe1 to θe3 of the rotor 20, which are calculated using the corresponding expressions (1) to (3), and the mechanical angle θ of the rotor 20 in which the electric angles θe1 to θe3 are plotted along the axis of the ordinate and the mechanical angle θ is plotted along the axis of the abscissas. Normally, the electric angles θe1 to θe3 change while being equal to each other. The controller 3 detects the electric angles θe1 to θe3 of the rotor 20, which are, in other words, the electric angle of the rotary shaft 1, in this manner.
In the rotational angle detection apparatus, the signals Va to Vc output by the resolver 2 have non-normal values when the apparatus has a malfunction such as a break in the wiring system of any one of the first to third resolver coils 23 to 25, a power fault, in which a wire contacts a power supply line, or a ground fault, in which a wire contacts a grounding wire. This hampers appropriate detection of the electric angles θe1 to θe3 of the rotor 20. Accordingly, if a malfunction such as a break in the wiring system in any of the first to third resolver coils 23 to 25 occurs, it is demanded that the malfunction be detected appropriately.
To meet this demand, as described in, for example, Japanese Laid-Open Patent Publication No. 2006-138778, a conventional rotational angle detection apparatus detects the aforementioned malfunction based on the sum of squares of the output signal amplitude values Sa, Sb, Sc. Specifically, the sum of squares S of the output signal amplitude values Sa to Sc is determined as indicated by the expression (4) described below.
                                                        S              =                            ⁢                                                Va                  2                                +                                  Vb                  2                                +                                  Vc                  2                                                                                                        =                            ⁢                                                                    (                                          K                      +                                              Vpp                        /                        2                                                              )                                    2                                ×                                  (                                                                                    (                                                  sin                          ⁡                                                      (                                                          θ                              ⁢                                                                                                                          ⁢                              e                                                        )                                                                          )                                            2                                        +                                                                  (                                                  sin                          ⁡                                                      (                                                                                          θ                                ⁢                                                                                                                                  ⁢                                e                                                            +                                                              120                                ⁢                                °                                                                                      )                                                                          )                                            2                                        +                                                                  (                                                  sin                          ⁢                                                      (                                                                                          θ                                ⁢                                                                                                                                  ⁢                                e                                                            +                                                              240                                ⁢                                °                                                                                      )                                                                          )                                            2                                                        )                                                                                        (        4        )            
Specifically, the relationship among the values sin(θe), sin(θe+120°), and sin(θe+240°) is represented by the expression (5) described below.(sin(θe))2+(sin(θe+120°))2+(sin(θe+240°))2=1.5  (5)
Accordingly, the sum of squares S is a fixed value represented by the expression (6) described below.S=1.5×(K×Vpp/2)2  (6)
As a result, if the electric potential difference between the peaks of the AC voltage Vr supplied to the excitation coil 22 is set to 4[V] and the transformation ratio is set to 0.2, for example, the sum of squares S is 0.24.
Specifically, the expression (6) is an ideal expression. That is, the sum of squares S is varied actually by various types of errors such as a detection error or a computation error. Accordingly, the controller 3 sets an upper limit threshold value greater than the theoretical value (0.24) and a lower limit threshold value smaller than the theoretical value. If the sum of squares S is either greater than or equal to the upper limit threshold value or smaller than or equal to the lower limit threshold value, the controller 3 determines that a malfunction such as a break has occurred in the wiring system of a resolver coil. In this manner, the malfunction is detected simply by comparing the sum of squares S of the output signals Va to Vc of the resolver 2 with the threshold values. This facilitates detection of a malfunction.
Malfunctions that occur in the wiring systems of the resolver coils 23 to 25 include, for example, a short circuit in the signal lines 23a to 25a corresponding to the resolver coils 23 to 25, in addition to the aforementioned break or ground fault. When a short circuit happens in any one of the signal lines 23a to 25a, it is difficult to detect the short circuit based on the sum of squares S of the output signal amplitude values Sa to Sc, as will be described in detail. Specifically, with reference to FIGS. 12, 15, and 16, the electric angle of the rotor 20 detected through the controller 3 at the time when a short circuit occurs in any signal line 23a to 25a will be described.
For example, as indicated by the double-dashed lines in FIG. 12, a short circuit may occur between the signal line 23a corresponding to the first resolver coil 23 and the signal line 24a corresponding to the second resolver coil 24. In this case, each one of the signals Va, Vb output from the corresponding output terminals Ta, Tb of the resolver 2 represents the average of the voltage induced in the corresponding one of the first and second resolver coils 23, 24. The waveforms of the output signal amplitude values Sa, Sb, which are detected by the controller 3, thus change from the shapes illustrated in FIG. 13 to the shapes illustrated in FIG. 15. In this state, the electric angles θe1 to θe3 of the rotor 20, which are determined by the controller 3 using the expressions (1) to (3), represent the values shown in FIG. 16. In other words, the calculated electric angles θe1 to θe3 represent either 30° or 210° and are greatly different from the actual electric angle of the rotor 20. FIG. 17A is a graph representing the relationship between the electric angle error Δθd and the rotor mechanical angle θ in which the electric angle error Δθd is plotted along the axis of the ordinate and the rotor mechanical angle θ is plotted along the axis of the abscissas. The electric angle error Δθd is obtained by subtracting the calculated electric angle θe1 at the time of a short circuit in the signal line represented in FIG. 16 from the calculated electric angle θe1 in a normal state represented in FIG. 14. Referring to FIG. 17A, a great error in the range of −90° to 90° is generated between the electric angle of the rotor 20 calculated at the time of a short circuit and the actual electric angle of the rotor 20.
When the output signal amplitude values Sa to Sc vary in the manner represented in FIG. 15, the sum of squares S of the output signal amplitude values Sa to Sc vary in a sinusoidal manner with respect to the mechanical angle θ of the rotor 20 as represented in FIG. 17B. Specifically, in this case, the lower threshold value Smin of the sum of squares S is set to (0.1176 (0.1176=0.24×0.72)) with respect to the theoretical value (0.24) and the upper threshold value of the sum of squares S is set to (0.4056 (0.4056=0.24×1.32)). FIG. 17B does not include the upper threshold value for the illustrative purposes. In this case, referring to FIG. 17C, the controller 3 is allowed to detect a malfunction when the sum of squares S is smaller than or equal to the lower limit threshold value Smin but cannot detect a malfunction if the sum of squares S is greater than the lower limit threshold value Smin. Accordingly, when the electric angles θe1 to θe3 of the rotor 20 are calculated in the rotational angle range A1 to A9 of the rotor 20 in which the sum of squares S exceeds the lower limit threshold value Smin, the controller 3 detects the calculated electric angles θe1 to θe3 as normal electric angles. As a result, an error between the electric angle of the rotor 20 that is erroneously detected by the controller 3 and the actual electric angle of the rotor 20 represents the values illustrated in FIG. 17D. If actuation of an electric motor is controlled based on an electric angle with such a great error in, for example, a power steering apparatus, behavior of the electric motor may disadvantageously change to a great extent.
This problem is not restricted to resolvers but commonly noted for a rotation sensor that outputs three-phase signals corresponding to the rotational angle of a rotor from three magnetic field change detecting sections, which are spaced apart in a circumferential direction of the rotor.